1. Technical Field
This invention relates to a method and apparatus for determining the rate at which the amplitude of an analog signal is sampled to achieve synchronization of the samples to a time base.
2. Background Art
At the present time, diagnosis of faults within a large electronic system, such as a telephone switch within a central office, is usually performed by a field technician, using one or more pieces of electronic test equipment, such as an oscilloscope. The technician uses the oscilloscope to observe the waveform of one or more signals of interest within the electronic system. From the waveforms displayed on his or her oscilloscope, the technician can often diagnose the fault and take appropriate corrective action.
In some instances, the technician may be unable to diagnose the fault from the waveforms displayed on his or her oscilloscope. When the technician cannot diagnose the fault, a troubleshooting engineer, who is often at a location remote from the site of the electronic system, is contacted, usually by telephone. The troubleshooting engineer receives a verbal report from the technicain of the symptoms experienced by the electronic system, and from that report, the engineer may make a diagnosis, or may suggest that additional tests be carried out to find the cause of the fault.
The ability of the troubleshooting engineer to diagnose the fault or suggest what additional tests should be carried out is dependent on how well the engineer has interpreted the information provided by the technician. If, for instance, the technician has not properly described the waveform observed on his or her oscilloscope, then the troubleshooting engineer may be unable to diagnose the fault. The troubleshooting enginer may then have to travel to the site of the faulty electronic system in order to observe, first hand, the symptoms exhibited thereby and to examine the waveforms displayed on the technician's oscilloscope. Such travel is time consuming and expensive.
As an alternative to sending the troubleshooting engineer to the site of the faulty electronic system, electrical signals from the faulty system may be transmitted to the engineer, typically over the telephone line. In this way, the troubleshooting engineer may directly observe the waveform of such signals on an oscilloscope, thereby avoiding the problem of misinterpretation of the waveform by the technician. In practice, the electrical characteristics of most if not all the signals of interest within the faulty electronic system are such as to prevent the transmission thereof directly over the telephone line without degradation.
To facilitate transmission of the electrical signals on interest from the site of the faulty electronic equipment to the site of the troubleshooting engineer, the signals from the faulty equipment are first digitized by sampling the amplitude thereof at intervals equally spaced in time. The amplitude of the signal at each interval is translated into a digital value which is stored at one of a plurality of successive locations in a memory at the site of the faulty equipment. The digital values within the memory are transmitted serially across the telephone line to the site of the troubleshooting engineer by the aid of a pair of modems (modulator/demodulators), each located at a separate end of the line. After transmission across the telephone line, each digital sample is stored in a memory located at the site of the troubleshooting engineer. The stored values are then successively converted into analog signals by a digital-to-analog converter. When these analog signals are sequentially applied to the vertical input of an oscilloscope, a waveform will be displayed thereby closely resembling that displayed by the technician's oscilloscope.
The electrical signals transmitted to the site of the troubleshooting engineer will not be of much assistance if the amplitude versus time relationship of such signals becomes distorted during the digitizing process. Furthermore, the sampling of the amplitude of the signals from the faulty electronic system must be synchronized to some periodically occurring event. Otherwise, the successive waveforms displayed on the oscilloscope of the troubleshooting engineer will not be in the proper phase or time relationship.
Heretofore, the problem of maintaining the amplitude versus time relationship of the signals transmitted to the troubleshooting engineer has been solved by sampling the signals at a fixed rate over a given period of time to obtain a fixed number of samples. The sampling rate is typically chosen twice as high as the highest frequency signal of interest within the electronic system to insure faithful reproduction of the waveform therof on the oscilloscope of the troubleshooting engineer. Initiation of the sampling occurs at the start of each waveform traced by the technician's oscilloscope. The rate at which each waveform is traced by the technician's oscilloscope (known as the horizontal sweep rate) can be varied.
Fixing the sample rate twice as high as the highest frequency signals of interest will produce more than the required number of samples needed to accurately reproduce signals of a lower frequency. Since the maximum rate at which the digital values can be transmitted across an ordinary telephone line without degradation is fixed (typically 1200 baud), the larger the number of samples that must be transmitted, the longer the transmission time. To reduce transmission costs, the amount of information transmitted should be minimized. For this reason, it is undesirable to excessively sample the signals of interest within the electronic system.
Accordingly, there is a need to provide a method for automatically establishing the optimum rate at which a signal is to be sampled within a preselected interval so that a preselected number of samples are taken.